a) Field of the Invention
Techniques for characterizing EUV radiation and beam profiles are needed in EUV lithography. These techniques pertain to the characterization of EUV sources directly as well as to the characterization of downstream beam delivery systems such as collector optics, illumination optics, imaging optics and masks. On the one hand, these components must be characterized with a view to meeting specifications; on the other hand, corresponding measurement methods are needed for process control within the total system. In this connection, the behavior of these components in the EUV in-band region of 13.5 nm is of particular interest. The present invention relates, in particular, to a narrow-band transmission filter for EUV radiation.
b) Descrption of the Related Art
Plasma lamps which emit in a broad spectral band from the soft x-ray range to thermal infrared are provided for generating the EUV radiation required for lithography. Therefore, one of the chief tasks of a method for measuring EUV radiation consists in the spectral filtering of the proportion of EUV in-band radiation which is in the range of several percent.
The use of reflection gratings and transmission gratings in monochromators and spectrographs in a known manner for spectral filtering (monochromatization) of EUV radiation requires well-defined beam paths which are usually given by entrance slits and exit slits. Not only does this lead to a relatively complex construction of high mechanical precision but also, due to the usually small effective grating surface, only a small solid angle of radiation can be detected. The latter is disadvantageous in extensively isotropic radiation sources such as plasma lamps because, for one, the flow of radiation to the detector is small and, for another, a measuring system based on gratings must be oriented precisely to the radiation source.
The reflectivity of multilayer mirrors with a corresponding layer design can present a pronounced narrow-band maximum at 13.5 nm. Therefore, they are extremely well suited for spectral filtering of EUV in-band radiation. Measurement arrangements based on this concept are described in the publication WO 03/017005 A1. However, the position and shape of the maximum of the reflectivity of multilayer mirrors depend not only on the layer design but also on the incident angle of the radiation. Consequently, the incident angle of the radiation must lie within determined tolerances in order to avoid an excessive change in the reflectivity as a function of wavelength. In practice, diaphragms are used for isotropic EUV sources in order to ensure a sufficiently parallel beam profile. The diaphragms define the detectable solid angle. In addition, the source, diaphragms, multilayer mirrors and detectors must be arranged on an axis.
Multilayer systems can also be used in transmission for spectral filtering. As in the reflection case, a narrow-band maximum can also be generated at 13.5 nm in transmission. While transmission is lower compared to the reflection of conventional multilayer systems, it is nevertheless adequate for applications with a sufficiently high radiation flow. The advantage results from the very simple construction of measuring devices because the transmission layer can be arranged directly on an EUV-sensitive detector (U.S. Pat. No. 6,710,351 B2).
However, the restrictions with respect to the incident angle region of the radiation are similar to those in the reflection case. These restrictions play only a subordinate role in spectral filtering by means of transmission layers which are based on the absorption characteristics of the elements or their connections. While the transmission changes with the incident angle of the radiation due to the changing effective layer thickness, this effect is minor in the case of near vertical incidence which is the relevant case in practice. More importantly, there is almost no spectral shift of the transmission as a function of the incident angle, which is not the case with multilayer mirrors. However, there are difficulties involved in realizing a sufficiently narrow transmission window.